Banana & walnuts cocoa cake

It is rare for me to bake a cake. I don't have a sweet tooth and am not good at making sweets. I think it is more difficult to make sweets than cooking. For cooking, I can taste sometimes and add more ingredients. But for sweets, I have to measure the exact ingredients, or the sponge of cake wouldn't be puffy sometimes.

Ingredients : I use 20 x 10cm heat resistant glass case to bake
1 egg - whisk well
100cc flour
1 tsp baking powder
1 tbs cocoa powder (I use nestle for chocolate drink)
50cc sugar
3 tbs butter - melt it in the microwave

1 banana - slice

some walnuts

Preparation :
1. Pre-heat oven 180C
2. Spread oil in the case and sprinkle walnuts
3. Mix flour, baking powder, cocoa powder and sugar in the bowl
4. Add egg into #2 and whisk well
5. Add butter into #3 and wisk well
6. Pour #4 into the case and put sliced banana
7. Put the case in the oven and bake for 20 - 30 minutes
8. Pierce a stick on the cake and if anything don't come out on the stick, it is done.

Note : It shows "cc" to measure because I don't have a scale.